Method for controlling and/or regulating the cooling stretch of a hot strip rolling mill for rolling metal strip, and corresponding device

ABSTRACT

The joint properties of a metal strip being rolled in a hot strip rolling mill, especially a steel strip, are adjusted in the cooling stretch of said mill by cooling. According to the invention, a time-related cooling course is predetermined for each strip point of the metal strip. An individual cooling curve is established as a function of time for each strip point, the established time curve is constantly compared with the model time-related cooling curve for each strip point and process control signals for controlling and/or regulating the cooling stretch are derived from this comparison. The corresponding device is provided with a calculating device and a process control device.

[0001] The invention relates to a method for the open-loop and/orclosed-loop control of the cooling section of a hot strip rolling millfor rolling metal strip, in which the microstructural properties of therolled metal strip, in particular a steel strip, are adjusted by thecooling. In addition, the invention also relates to the associateddevice for carrying out the method.

[0002] In the steel industry especially, so-called slabs are rolled inthe hot state into strips in a hot strip rolling mill. After rolling,the metal sheet runs through a cooling section. The cooling section ofthe hot strip rolling mill serves for adjusting the microstructuralproperties of the rolled steel strips.

[0003] The microstructural properties of the strips produced havepreviously being derived predominantly from the coiling temperature,which is kept constantly at a specifed setpoint value by the coolingsection automation.

[0004] New materials, such as multiphase steels, TRIP steels or thelike, require a precisely defined heat treatment, i.e. the specificationand monitoring of a temperature profile from the last rolling stand tothe coiler.

[0005] “Proceedings of ME FEC Kongreβ 99”, Dusseldorf, June 13-15, 1999(Verlag Stahl Eisen GmbH) discloses a proposal for the automation of hotstrip rolling mills in which model-supported control is providedspecifically for the cooling section. In this case, the cooling is basedon the idea that a reference temperature can be specified over thelength of the entire cooling section and that the temperature measuredat a particular time is adapted to the specified values by means of anadaptive control unit. What is important in this case is that theinfluence of the cooling can be registered in the longitudinal andvertical directions by means of enthalpy observations and dividing thecooling process into a series of smaller thermodynamic processes. Inparticular, this involves calculation by means of the method of “FiniteElements”.

[0006] On the basis of the latter, it is the object of the invention tospecify an improved method for the automation of cooling sections in hotstrip rolling mills and to provide the associated device.

[0007] The object is achieved according to the invention by thecharacterizing features of patent claim 1. Developments are specified inthe dependent claims. An associated device for carrying out the methodis characterized by the features of claim 10.

[0008] The problems presented at the beginning are now solved not in thesame way as in the prior art by specifying the temperature profile alongthe cooling section but by specifying an individual course of coolingover time for each strip point of the metal strip. What is particularlyadvantageous about this is that such a specification can be determineddirectly from the desired properties of the steel and remainsindependent of variable process values, such as for example the speed ofthe strip.

[0009] Consequently, in the case of the method according to theinvention it is important that, for each so-called strip point of thematerial to be cooled, an own course of cooling over time is specified.Consequently, the time functions determined in this way can be comparedat any time for any strip point with the specified time-based coolingcurves.

[0010] The method according to the invention has the advantage thatcooling conditions which correspond better to the actual conditionsdictated by practical circumstances can be specified. It is nowadvantageously possible for variable cooling along the strip also to bespecified, whereby regions of specific quality can be produced in therolled strip in a specifically selective manner. As a result, so-calleddual-phase materials can also be produced, which was not possible in theprior art.

[0011] The fact that the course of cooling is specified for each strippoint along the entire cooling section means that the open-loop and/orclosed-loop control is no longer tied to fixed switching locations;rather, any desired valves for supplying coolant can be actuated at anytime. In order that it is possible for maintenance of the specifiedcooling along the cooling section to be checked by the open-loop and/orclosed-loop control, according to the invention a model is calculated inreal-time along with the strip in the cooling section. This provides therequired strip temperatures on the cooling section and is constantlycorrected by measured temperature values.

[0012] The method according to the invention consequently allowsaltogether a flexible specification of the heat treatment for modernsteels. This means that practical requirements are met.

[0013] In the case of corresponding devices, which respectively includea cooling section which can be subjected to coolants over its entirelength by respectively individually adjustable valves, there are meansfor specifying cooling curves for the individual strip points of themetal strip. There are also units for calculating the cooling curves,for correcting the determined cooling curves on the basis of measuredtemperatures, for comparing with the specification of the cooling curvesand for generating process control signals. These units can beimplemented in a computer by means of software.

[0014] Further details and advantages of the invention emerge from thefollowing description of the figures depicting exemplary embodiments onthe basis of the drawing in conjunction with further subclaims. In thedrawing:

[0015]FIG. 1 shows the construction of a cooling section arrangeddownstream of the rolling mill,

[0016]FIG. 2 shows a three-dimensional temperature-time/strip-lengthdiagram,

[0017]FIG. 3 shows the structural diagram of the open-loop/closed-loopcontrol, including model correction for the cooling section according toFIG. 1, and

[0018]FIG. 4 shows specifically the calculation of the model correctionfrom FIG. 3.

[0019] The cooling of metal strip as part of hot rolling technology andspecifically the function of the cooling section in this technology isillustrated on the basis of FIG. 1. In the hot rolling of steel,so-called slabs with an initial thickness of about 200 mm are rolledinto a strip of 1.5 to 20 mm. The processing temperature is in this case800 to 1200° C. The end of the process after rolling includes coolingthe strip with water in a cooling section down to 300 to 800° C.

[0020] In FIG. 1, the last rolling stand of a hot strip rolling mill isdenoted by 1. The rolling stand 1 is followed by a finishing-trainmeasuring station 2 and after the cooling there is a coiler measuringstation 3, in which stations the temperature of the strip is measured,and after that there is an underfloor coiler 4 for winding up the metalstrip into a coil. Between the finishing-train measuring station 2 andthe coiler measuring station 3 there is the cooling section 10, which inthe present context is generally referred to as a system.

[0021] A rolled hot strip of steel is denoted in FIG. 1 by 100. It runsthrough the cooling section 10 and is cooled on both sides by means ofvalves with a cooling medium, in particular water. Individual valves canbe combined into groups, for example the valve groups 11, 11′, . . . ,12, 12′, . . . , 13, 13′, . . . and 14, 14′, . . . are represented.

[0022] The cooling of the strip 100 to be registered by closed-loopcontrol is usually based on a one-dimensional non-steady-state heatconduction equation. The mathematical description is based on aninsulated bar which undergoes a heat exchange with the ambience only atthe beginning and end—corresponding to the upper side and underside ofthe strip.

[0023] For the heat conduction in the strip especially, the modelassumption that the heat conduction system diminishes to nothing in thelongitudinal and transverse directions and that the enthalpy is constantover the width of the strip is taken as a basis. As a result, theproblems can be reduced to a one-dimensional non-steady-state heatconduction problem, in which the initial conditions and the boundaryconditions have to be defined.

[0024] On the basis of the latter model, the strip 100 can be describedby individual strip points, in which a heat conduction takes place inthe bar. This is known, in respect of which reference is made to therelevant technical literature.

[0025] Generally, no temperatures can be measured in the cooling section10. However, the temperature is measured at the measuring station 2upstream of the cooling section and in particular at the coilermeasuring station 3. The heat exchange in the strip 100 is taken intoaccount in the mathematical model in accordance with the abovepreconditions. Consequently, a model of the cooling section, which isdenoted in FIG. 1 by 15, is created. When the temperatures are availableat any desired point via the model 18, closed-loop control to thespecified cooling profile can be realized.

[0026] The specification of a course of cooling is represented in FIG. 2on the basis of a three-dimensional temperature strip-length/timediagram:

[0027] Proceeding from a beginning of cooling (t=0) of a strip point, aspecified cooling profile 300 is obtained over the time t as a timefunction. FIG. 2 reveals for each strip point of the metal strip 100 anown cooling curve. For example, the curve 300 for a specific strip pointat li is represented, an own time function being obtained in this wayfor this strip point.

[0028] For example, the temperature profile for the strip point i aftera specific cooling time t_(i) is intended to have a specifiedtemperature T_(i), in particular coiling temperature T_(H). There arealso corresponding specifications for the remaining strip points. If allthe specified coiling temperatures of the individual strip points arejoined, the curve 400 depicted in FIG. 2 is obtained. With this curve400, it can be ensured for example that method steps such as seizing thestrip at the coiler with otherwise the least possible microstructuralchanges are taken into account.

[0029] If at one instant the specifications of all the strip pointslying in the cooling section 10 at the time are then considered andthese strip points are joined, a curve 500 which represents the coolingprofile over the length of the cooling section is obtained. This coolingcurve is also depicted in FIG. 1 in unit 30. What is important here isthat, according to the specified technical teaching, the curve 500 isdynamically adapted automatically when there are disturbances in theproduction process, for example when there is a variable strip speed. Asa result—by contrast with the prior art—such disturbances remain withoutany effects on the specified course of cooling of each strip point.

[0030] It is consequently important in the case of the method describedthat, for each strip point, own cooling curves 300, 310, 311, 312 etc.are specified. For example, for the first point, a cooling curve with aninitially steep descent and subsequently a flatter descent is specified,whereas in the middle region cooling curves with virtually constanttemperature gradients are obtained. Consequently, the described profile400 is achieved overall.

[0031] Other cooling profiles can also be produced. In particular, ifthe microstructure is taken as a basis as a target variable, the profilecan be specified in such a way that there are, as far as possible,constant microstructural properties on the finished strip.

[0032] However, a change in the microstructural properties can also bedeliberately provided for specific regions of the strip. For example,microstructural changes caused by the greater lying time of the rearportions of strip can be offset again before further rolling.

[0033] Since the microstructural properties determine the mechanicalproperties and consequently the quality, in particular of steel strip,desired material properties can be accomplished by specificallyselective microstructural changes. To this extent, the method describedprovides increased potential in the production of finished strip.

[0034] In FIG. 3, the cooling section is denoted by 10 as an actualsystem. The model forming of FIG. 1 is expressed here by a so-calledreal-time model 20, by means of which the temperatures {circumflex over(T)}_(i) at the individual strip points i of the strip 100 aredetermined.

[0035] The calculated coiling temperature {circumflex over (T)}_(H),which is affected by an error, is compared with the temperature T_(H)measured at the coiler 3 and the resulting error is fed to a unit 25 formodel correction. The latter unit 25 is also fed the entire coolingprocess 3, calculated from the real-time model 20. The unit 25determines from these data a correction of the course of cooling, whichis applied to the calculated course of cooling. The corrected course ofcooling determined in this way is compared with the setpoint cooling andthe resulting system deviation is fed to the controller 30. The latterproduces from this and by means of the gains determined from the unit 25the valve settings as process control signals, which are both convertedon the system and fed again to the real-time model 20 as information.

[0036] If no valid measured value is available, the calculation of acorrected course of cooling does not take place. The correction is thenassumed to be zero.

[0037] The controller 30 can be operated on the basis of the enteredsystem deviation and the further values with a specified algorithm. Suchalgorithms are specified by means of software and allow the activationof any desired specimens of valves. In particular, with the controllereach of the valves 11, 11′, . . . , 12, 12′, . . . , 13, 13′, . . . ,14, 14′, . . . can be simultaneously activated at any time in anydesired combination by the controller

[0038] The cooling along the metal strip is specifically observed on thebasis of the enthalpy and the temperature variation as a function of theenthalpy.

[0039] In FIG. 4, the calculation of the model correction for thecontroller is specifically illustrated: the enthalpies e and thetemperatures T are determined as a function of the enthalpy e. Thereal-time model 20 provides a calculated enthalpy value ê, from whichthe value {circumflex over (T)} (ê) is formed in a unit 21. Thisconsequently allows the temperature values {circumflex over (T)} to becalculated for any desired strip points. To be specific, the calculatedtemperature value {circumflex over (T)}_(H) for the coiling temperatureis compared with the measured coiling temperature T_(H), from which avalue ΔT_(H) is obtained.

[0040] From the real-time model 20, enthalpy signals are likewise fed toa unit 22, in which the partial derivative of the enthalpy is formed onthe basis of the heat conduction coefficient$\frac{\partial\hat{e}}{\partial\kappa}.$

[0041] To a certain extent, the heat conduction coefficient represents acorrection factor. The valve settings of the system are also entered inboth units 20 and 22.

[0042] Calculated values $\frac{\partial\hat{e}}{\partial\kappa}$

[0043] are obtained as the output signal of the unit 22. In unit 23,$\frac{\hat{T}}{\hat{e}}$

[0044] is applied to the signal, allowing a signal$\frac{\partial\hat{T}}{\partial\kappa}$

[0045] to be determined by the forming of partial derivatives on thebasis of the chain rule.

[0046] The value for the coiler$\frac{\partial{\hat{T}}_{H}}{\partial\kappa}$

[0047] especially is considered and the previously determinedtemperature error ΔT_(H) is divided by this value, producing the Δκ. Thelatter value Δκ is multiplied by$\frac{\partial\hat{e}}{\partial\kappa},$

[0048] so that the model correction Δe is obtained as the output value.This gives the model correction of the unit 25 from FIG. 3.

[0049] In the calculation of the model correction Δe according to FIG.4, $\frac{\partial\hat{e}}{\partial\kappa}$

[0050] consequently represents a sensitivity model

[0051] It has been found that, with the above procedure andconsideration of the cooling curves for the individual strip points, theconditions for practical circumstances can be modeled better. In thiscase, the procedure is based on the realization that the heat treatmentof modern steels can be individually specified by directly specifyingthe setpoint curves for the temperature profile of the actual course ofcooling for each strip point. To this extent, the interface for theopen-loop and/or closed-loop control is the model calculated in realtime and the associated correction algorithm constitutes an essentialpart of the method described.

[0052] This procedure takes the specification for the finished materialinto account in an ideal way, since it ensures the adjustment of therequired quality within the limits of the system—independently of thestrip speed used.

1. A method for the open-loop and/or closed-loop control of the coolingsection of a hot strip rolling mill for rolling metal strip, inparticular a steel strip, the microstructural properties of the rolledmetal strip be adjusted by cooling, with the following method steps: foreach strip point of the metal strip, a course of cooling over time isspecified, in addition, for each strip point of the metal strip, theactual cooling curve is determined as a function of time, the determinedtime function of the actual course of cooling is compared with thespecification of the course of cooling over time for each strip point ofthe metal strip; process control signals for the open-loop and/orclosed-loop control of the cooling section are derived from thedeviations of the determined time curves from the actual course ofcooling.
 2. The method as claimed in claim 1, characterized in thatdifferent cooling curves are specified for individual strip points ofthe metal strip.
 3. The method as claimed in claim 1 or claim 2,characterized in that desired microstructural properties are adjusted onthe basis of the specified cooling curves for each strip point of themetal strip.
 4. The method as claimed in claim 3, characterized in thatsuch cooling curves that undesired changes in the microstructuralproperties occurring on account of external influences are offset arespecified for the individual strip points of the metal strip.
 5. Themethod as claimed in claim 3, characterized in that the cooling curvesfor the individual strip points of the metal strip are specified in sucha way that predetermined, possibly different, microstructural propertiesare obtained for different strip points of the metal strip.
 6. Themethod as claimed in claim 5, characterized in that the mechanicalproperties of the metal strip are specified on the basis of thespecifically selective influencing of the microstructural properties. 7.The method as claimed in one of the preceding claims, characterized inthat the time functions or individual values at the given instant intime of the course of cooling of individual strip points are fed to acontroller and lead to the generation of the process control signals. 8.The method as claimed in claim 7, it being possible to use thecontroller for activating valves for coolant for cooling the metalstrip, characterized in that any desired valves can be simultaneouslyactivated by the controller at any point in time.
 9. The method asclaimed in one of the preceding claims, characterized in that themeasured time function of the coiling temperature is used as thecomparison temperature with respect to the cooling curves of individualstrip points.
 10. A device for carrying out the method as claimed inclaim 1 or one of claims 2 to 9, with a cooling section, in which themetal strip running through can be subjected to coolant by means ofadjustable valves (11, . . . , 13), and a unit for determining thetemperature-time functions of each individual strip point of the metalstrip and with a process control unit (30) for obtaining process controlsignals for the open-loop and/or closed-loop control of the cooling inaccordance with specified criteria.
 11. The device as claimed in claim10, characterized in that, with the process control unit (30), each ofthe individual valves (11, 11′, . . . to 13, 13′, . . . ) for supplyingcoolant can be activated at any time.
 12. The device as claimed in claim10, characterized in that the criteria comprise a cooling profile alongthe metal strip in accordance with desired microstructural properties.13. The device as claimed in claim 10, characterized in that the processcontrol unit for the open-loop and/or closed-loop control of the coolingis based on a real-time model (20) with a model correction (25), fromwhich the input signals for a controller (30) for activating theindividual valves (11, 11′, . . . to 14, 14′, . . . ) are derived. 14.The device as claimed in claim 10, characterized in that the measuredcoiling temperature (T_(H)) is used for the model correction.
 15. Thedevice as claimed in claim 10, characterized in that the systemdeviation for the controller (30) is formed from a corrected course ofcooling and the setpoint cooling.